A solder paste is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly referred to as a solder joint. Solder pastes generally comprise powdered metallic solders of various alloys such as tin-lead, tin-lead-silver, tin-lead-bismuth, tin-lead-antimony, tin-silver etc. contained in a vehicle including a flux. In addition a suspension medium, such as hydrogenated castor oil, may be present in the vehicle and combined with the flux to provide a solder paste of uniform texture and consistency.
In electronic circuity, solder pastes are employed to secure miniature electronic components to a circuit on a substrate such as ceramic or glass or the like. Circuits, similar to those used on printed circuit boards are produced on the substrates. The areas of the circuit to which electronic components are to be added are commonly referred to as pads. Generally in a typical surface mount electronics manufacturing process, a solder paste is screen or template printed onto the pads of the printed circuit board. Next, the electronic components are carefully positioned with their peripheral contacts in contact with the solder paste coated pads.
When all of the electronic components are in place on the circuit board, the solder paste is caused to reflow. One way the solder paste can be caused to reflow by passing the board through an infrared belt furnace. The heat generated in the infrared furnace causes the solder paste to reflow thus fusing the electronic components to the circuit board.
Flux residues are also generated by the reflowing of the solder paste. The flux residues are generally corrosive and therefore are generally removed from the circuit board. Typically, the removal of the flux residues is accomplished by washing the printed circuit board in a CFC based solvent.
After the flux residues are removed the circuit board is generally suitable for use. However, to protect the solder joints from the corrosive effects of elements such as moisture and salt air, an epoxy coating may be applied over the solder joints. Generally this epoxy coating is applied in liquid form and then cured to form a clear, hard protective coating.
A problem, as set forth above, with generally utilized solder pastes is that the reflowing of the solder paste creates corrosive flux residues. A related problem is that the removal of flux residues creates a step in the surface mount electronic manufacturing process. Since the removal of the flux residues is generally accomplished by washing the printed circuit board in a CFC (chlorinated-fluorocarbon) based solvent, and CFC solvents present environmental hazards in their use and disposal, the removal of flux residues in addition to creating an extra step, may cause hazardous conditions in the workplace.
A further problem in generally performed surface mount electronic manufacturing processes is that the protective epoxy coating, if desired, must be applied in an additional step.
It would be advantageous in a surface mount electronic manufacturing process to have a solder paste which did not produce corrosive flux residues when reflowed.
It would also be advantageous in a surface mount electronic manufacturing process to have a solder paste which could be cured to form a protective epoxy coating on the solder joints without the need for an additional coating step.